Jean Jacques Ehrhardt

A study of the adsorption of several oxygen-containing molecules (O2, CO, NO, H2O) on Re(0001) by XPS, UPS and temperature programmed desorption

Abstract We have studied the adsorption of oxygen, carbon monoxide, nitric oxide, and water vapour on Re(0001), using X-ray and ultra-violet photo electron spectroscopies (XPS and UPS) and temperature-programmed desorption. As on polycrystalline rhenium, adsorbed oxygen is completely dissociated, even at room temperature. Furthermore, the formation of a superficial oxide at room temperature seems probable. Carbon monoxide is almost completely molecularly adsorbed, only a very small fraction being dissociatively adsorbed in a single β- state. However, an attractive interaction still exists between the adsorbed atoms in this β- state. Nitric oxide is adsorbed in a dissociated β 2 state and a molecular β 1 state. The population is smaller than on polycrystalline rhenium, corresponding to half a monolayer. Mathematical treatment of the desorption spectra allowed us to determine the activation energy for desorption of nitrogen resulting from the decomposition of adsorbed species. These quantities were found to be similar to those measured for polycrystalline rhenium.

Catalytic activation of cobalt induced by oxidizing treatments in the methanation of carbon dioxide

A ribbon of Co catalyzing CO2 hydrogenation at atmospheric pressure and temperatures ranging from 200 to 500 °C has been taken as an example for studying the activating effect of pre-oxidation upon the activity of metals in hydrogenation reactions. The chemical state of the Co surface could be determined at any time by direct and quick transfer of the sample from the reactor into the ultrahigh vacuum chamber of an electron spectrometer. After cleaning and prolonged exposure to H2 (15 h, 1 bar, 500 °C) the sample displayed no visible activity up to 500 °C despite the absence of any visible contaminant on its surface. Activities ranging over several orders of magnitude could be induced in the metal by adequate pre-oxidations according to their severity. A large part of this activation was transient and its progressive decay required periods of time which were longer the more severe the pre-oxidation and the lower the temperature of reaction. The activity at any time after treatment was shown to depend not only on the oxidation but also on the subsequent reduction, since an increase of the rate of reduction resulted in a corresponding though transient increase of the activity. Reduction of most of the surface region was shown to occur very quickly, whereas deeper layers continued to reduce during the progressive decay of the activity. No visible contamination occurred during deactivation. The activation is interpreted in terms of the creation of surface defects by the alternate oxidations and reductions whereas the deactivation is considered to originate in the thermal restructuring of the surface.

Deposition by laser ablation and characterization of titanium dioxide films on polyethylene-terephthalate

Abstract Thin titanium dioxide films have been deposited on polyethylene-terephthalate (PET) by laser ablation deposition technique. The experiments were performed with a quadrupled Nd:YAG laser (266 nm) and an ArF excimer laser (193 nm), at room temperature and without any oxygen addition during the ablation. The as-prepared films have been characterized by various techniques: Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), X-Ray Diffraction (XRD), X-Ray Microprobe, Laser Microprobe Mass Spectrometry (LMMS), and ellipsometry. For this last technique the titanium dioxide films were deposited onto a silicon substrate. The thicknesses of the films are ranging between 100 and 150 nm. Their surfaces are smooth and the presence of some holes (diameter

Coadsorption de l'oxygène et du monoxyde de carbone sur des surfaces de rhénium

Abstract Temperature programmed desorption ( 2.65 K sec ) has been used to study carbon monoxide and mixed layers of carbon monoxide and oxygen on rhenium ribbons, strongly oriented parallel to the (0001) plane. Four binding states, populated in decreasing energy have been detected. Interpretation of the results on β states agrees qualitatively with Kings model postulating dissociation of carbon monoxide molecules and a repulsive interaction energy between carbon and oxygen atoms. However, in the coadsorbed layers studies, it is shown that all the oxygen atoms do not play a part in the recombination process, during desorption, and that when oxygen is adsorbed after carbon monoxide, a displacement reaction occurs, due to apparent transfer from β states towards molecular α states. Optimization of the results on pure carbon monoxide layers leads to an interactional energy ω, equal to 3 kcal mole , and is only possible if is assumed that β states are formed on alternatively filled and empty rows.

Hydrogen and deuterium adsorption on polycrystalline rhenium surfaces; effect of carbon and oxygen coadsorption

Abstract Hydrogen adsorption has been studied on polycrystalline rhenium surfaces (films and ribbons) by uptake measurements and temperature programmed desorption. The total amount adsorbed Q and the sticking coefficient σ are lower on films than on ribbons. An isotopic effect has been observed on both surfaces where the sticking coefficient σ for D 2 is lower than the sticking coefficient for H 2 but the total amount adsorbed is the same for H 2 and D 2 on each surface: Q H 2 ribbon = Q D 2 ribbon = 6 × 10 14 atoms cm 2 , Q H 2 film = Q D 2 film = 3.5 × 10 14 atoms cm 2 , σ H 2 ribbon = 0.4, σ H 2 film = 0.1, σ D 2 film = 0.08. On ribbons thermal desorption spectra show that a part of the hydrogen is adsorbed as atoms with 67 kcal/mole for the binding energy with the surface at zero coverage, the other part is adsorbed as molecules with a binding energy of 27.5 kcal/mole Coadsorption of hydrogen with carbon or oxygen shows that these states probably correspond to different crystallographic sites on the surface.

Surface spectroscopic study of the adsorption of Ni(II) on pyrite and arsenopyrite at pH 10

The sorption mechanism of Ni(II) on pyrite (FeS 2 ) and on arsenopyrite (FeAsS) has been investigated at pH 10, using complementary methods for surface characterization such as x-ray photoelectron spectroscopy (XPS), Fourier transform mass spectrometry (FTMS), high-frequency dielectric measurements and electrophoretic mobility measurements. At this pH, the surfaces of the minerals are rapidly covered by an oxidation layer composed mainly of Fe(III) oxides on FeS 2 and Fe(III) arsenite and arsenate on FeAsS and presenting negative zeta potentials of -40 and -60 mV, respectively. The first step of the interaction with Ni(II) is the formation of a hydroxylated surface complex through proton exchange with the surface hydroxyl groups (≡Fe-OH). Then, electrostatic interactions between colloidal Ni(II) and the surface produce a heterogeneous coating of an average thickness of a few nanometres. This film inhibits significantly the oxidation by H 2 O 2 of these two minerals, particularly FeAsS.

Adsorption of xenon on oxidized Ni(100) and Ni(111) surfaces by leed and photoemission

Abstract Xenon adsorption has been studied by LEED and photoemission for the systems O Ni(100) and O Ni(111) in a large range of oxygen exposures from the chemisorbed phases to the first stages of oxidation. Xenon adsorption on the ordered oxygen phases evidences the local character of this probe. In the first stages of oxidation, this method confirms the existence of oxide islands even though no diffraction pattern is seen. Two kinds of oxide are clearly distinguished: a surface oxide, dominated by nickel-nickel interactions, and similar to non-stoichiometric oxides, and another oxide with dominant Ni-O interactions whose behaviour more closely resembles bulk oxide.

Lead deposition onto a polycrystalline platinum surface: I. Study by Auger electron spectroscopy and line of sight mass spectroscopy

Abstract Progressive lead deposition onto a platinum substrate at 300 K (as analysed by Auger electron spectroscopy and thermal desorption) corresponds to a two-stage mechanism. The most probable explanation is an adsorption of lead atoms in the first “layer” (almost covering the surface), followed by a lead-rich platinum alloy formation. These two types of surface present different catalytic properties, as it will be shown in following papers.

Adsorption of xenon on Ni(111) studied by photoemission and LEED

Abstract Adsorption of xenon on Ni(111) has been studied by LEED and by partly integrated photoemission UPS Hel at a polar angle of detection from 0° to 30°. An incommensurate structure has been evidenced at monolayer completion. For xenon coverages slightly lower than one monolayer complex LEED patterns have been obtained. They have been interpreted in terms of coexisting Xe p(2 × 2) domains and Xe hexagonal domains. Xe5p 1 2 and Xe 5p 3 2 splitting into two components has been observed in both first and second monolayers when the photoelectrons are detected along the normal to the surface. Adsorption on two different types of sites and indirect transition processes have been considered to interpret this splitting. The angular behaviour of the photoelectron spectra strongly supports the latter interpretation. A xenon bandwidth of 0.3 eV has been deduced. This value is fully consistent with the XeXe distance of 0.44 nm deduced from LEED for the incommensurate structure.

The Eu/Pd(111) interface: spectroscopic and structural studies

Abstract Growth of Eu ultra thin films on a Pd(111) single crystal, kept at room temperature, has been studied by spectroscopic (XPS, UPS, AES) and structural (RHEED) methods. Eu atoms of the first two deposited monolayers are divalent and form a p(2 × 2) arrangement on the Pd(111) surface. During the completion of the third Eu monolayer, this ordered structure smears out in connection with the appearance of trivalent Eu atoms at the interface. The fine structure modifications of Pd and valence band XPS spectra indicate the 4d band filling, via a charge transfer mechanism, of the Pd atoms involved in the interface. For intake Eu coverages a diffusion process leads to formation of disordered divalent EuPd alloys. After annealing the sample at 820 K, RHEED diffraction patterns reappear (p(2 × 2)) and, simultaneously, an increase in the Eu mean valence is observed. XPS results show that segregation of divalent Eu occurs at the surface of the new ordered bulk-trivalent EuPd compound which forms at the interface. This behaviour occurs whatever the thickness of the Eu deposit is.